Polypropylene materials which have been modified to improve resistance to impact are generally referred to as impact-modified polypropylenes. Heretofore, impact resistance of polypropylene has been improved by blending impact resistant materials such as elastomeric ethylene-propylene copolymers therewith or by reacting therewith other polymers or copolymers to produce reactor blends typically referred to as impact copolymers (ICPs). There is, however, a tradeoff with other properties, e.g., stiffness and flow characteristics, when such polypropylene materials are impact-modified. For example, U.S. Pat. No. 4,459,385 discloses that impact-modified polypropylenes obtained by sequential polymerization of propylene, and ethylene and propylene, to obtain an impact copolymer of polypropylene homopolymer and elastomeric ethylene-propylene copolymer, while having good impact resistance at low temperatures, have decreased stiffness. This reference suggests that as a general rule, impact resistance increases and stiffness decreases with increasing amounts of elastomer in the total composition; and that the decrease in stiffness can be compensated for by blending with the impact-modified material a linear low density polyethylene (LLDPE) having a melt flow index not exceeding 5 dg/min.
U.S. Pat. No. 4,535,125 discloses that impact-modified polypropylenes blended with a LLDPE have poor flow performance and that such flow performance is improved through visbreaking. That is, the melt flow rate of the blend of impact copolymer and LLDPE can be increased by extruding the blend in the presence of certain peroxide materials. Thus, impact-modified polypropylenes are blended with a LLDPE having a melt flow index of from 5 to 50 dg/min and the blends are visbroken to obtain impact-resistant polypropylene materials which have good stiffness characteristics as well as good flow characteristics.
Other methods of preparing impact-modified polypropylenes are also known. For example, U.S. Pat. No. 4,375,531 discloses blends of a first component selected from a group of medium impact polymeric materials with a second component selected from a group of high impact polymeric materials which blends are visbroken to improve the flow characteristics thereof.
Traditional ICP's utilizing metallocenes can be prepared as described in U.S. Pat. No. 7,122,498. These ICP's are first subjected to propylene to provide polypropylene resin particles. The polypropylene resin particles are then subjected to an ethylene/propylene gas mixture to form ethylene propylene rubber in the pores of the polypropylene resin particles. However, ICPs performance properties have been inhibited by the inability to increase the amount of rubber or ethylene within the ICP. Attempts to increase rubber content beyond 30 weight percent in many cases result in severe operability issues in the reactors. Furthermore, ICPs prepared with single site catalysts have been limited due to the low pore volume capability of metallocene-based iPP.
Further, EP 1 041 090 and WO 02/46251 disclose multistage polymerizations where two ethylene polymers are made in multiple stages.
Other references of interest are U.S. Pat. No. 5,066,723 and US 2014/0121325 A1.
In summary, there is a need to prepare ICPs with rubber content greater than 30 weight percent. Ideally, the process to prepare the ICP would not foul the reactor during production.